PURPOSE: To prevent malfunction due to mixing of noise from an input side, by using two photocouplers and keeping either of them to be turned on.

CONSTITUTION: When an input signal goes to "L", the input of a photocoupler 1 goes to "H" level with an NOT circuit 3, the output of the photocoupler 1 goes to "H" level and the output of an NOT circuit goes to "L" level. On the other hand, since the input of a photocoupler 2 is given to the photocoupler 2 via a buffer circuit 5, the input of a buffer circuit 6 and an AND circuit 7 goes to "L" level. Thus, the output of the AND circuit 7 goes to "L" level. When the input signal goes to "H" level, two inputs to the AND circuit 7 go to "H" level and the output of the AND circuit 7 goes to "H" level.